Method and device for the continous production of organic mono or polyisocyanates

ABSTRACT

Starting material streams are mixed in a mixer ( 13 ) for the phosgenation of amines by a process in which the reaction product ( 3 ) is removed in a closed loop and the starting material streams ( 1.2 ) contain organic solvents. The main streams ( 1.1, 2.1 ) and/or part-streams ( 1.2, 2.2 ) of the starting materials ( 1, 2 ) come into contact with one another according to the countercurrent principle.

[0001] The present invention relates to a process and an apparatus forthe continuous preparation of organic mono- or polyisocyanates byreacting the mono- or polyamines corresponding to the mono- orpolyisocyanates with phosgene at elevated temperatures, it beingpossible for the amines or the phosgene to be present in solution in anorganic solvent.

[0002] DE 2 153 268 relates to a continuous prephosgenation process forthe preparation of organic isocyanates. A diamine solution and phosgenesolution in the turbulent state are mixed continuously in a drivencentrifugal pump. The phosgene solution is introduced into thecentrifugal pump through the suction connection of the multistagecentrifugal pump and the amine solution is introduced into the lateralaccess additionally mounted in the middle between the first and thesecond impeller, before the prephosgenation mixture is transported bythe multistage centrifugal pump into a downstream hot-phosgenationstage.

[0003] EP 0 291 891 B1 relates to a process for the preparation ofisocyanates. Solutions and suspensions of primary amines and their saltsare mixed with phosgene solutions and reacted, the two substances beingintroduced into a mixing zone which has at least one rotor disk. Theresulting precursor is removed and the further reaction of the primaryproducts formed is effected with heating. When carrying out the mixing,the phosgene solution is fed in axially relative to the rotor disk andthe dissolved amine is sprayed parallel to the stream, but at a distanceaway from the stream of the phosgene solution, toward the rotor disk.

[0004] During the mixing processes with moving parts, which include thesolutions described above, the bearing points of the moving parts are asource of potential danger owing to the high toxicity of the phosgene,since the phosgene can escape through said points in the event of leaks.Attempts were therefore made to find a procedure to achieve mixing ofmono- or polyamines without moving parts.

[0005] EP 0 322 647 B1 discloses a process for the continuouspreparation of mono- or polyisocyanates, in which, for the preparationof the starting mixtures, the amine component, which if required isdissolved in an inert solvent, and the phosgene solution are combined ina nozzle by constricting one of the two components in this nozzle andfeeding the other component from the side, into this constriction, tothe stream of the first component in a plurality of part-streams througha corresponding number of holes distributed over the circumference ofthe constriction. The total length of the constriction is chosen so thatit comprises a part-length in which the reaction of the free amine isessentially complete. The disadvantage of this arrangement is that verysmall solid deposits in individual holes can lead to a lower flow ratethrough them.

[0006] DE-A1 29 50 216 discloses a process and an apparatus forthoroughly mixing two liquid components. The first component isintroduced under pressure in the form of a fan-like jet into anessentially cylindrical mixing chamber, flowing along its longitudinalaxis. Perpendicularly to this, the second component is introducedsimultaneously under pressure in the form of at least two fan-like jetsinto the jet of the first component, in its flow region. The resultingmixture of the two liquid components is then transported from the mixingchamber into a downstream reaction zone. However, the process appearsunsatisfactory owing to the high preliminary pressures required for theprocess.

[0007] SU 519 129 shows a production process for the preparation ofisocyanates. A production process for isocyanates is presented in whichgaseous phosgene is fed to a reactor at its bottom with a temperaturbetween 100° C. and 180° C. The phosgene runs into an amine salt, whichis fed to the reactor in its upper region. The amine salt is fed to theupper region of the reactor at a temperature between 40° C. and 100° C.

[0008] A Venturi mixing device is known from U.S. Pat. No. 3,507,626.This mixing device is designed for mixing a phosgene with an amine forthe preparation of isocyanates. It comprises a first and a second inletand an outlet. A first part of a pipe comprises a Venturi section with aconverging part, a constriction and a diverging part. A second part isplaced coaxially in the first part of the pipe and serves as a firstinlet. The second part of the pipe comprises a bevel which correspondsto the converging part. The second part of the pope runs into a mixingchamber, which extends around the Venturi section of the first part ofthe pipe. The mixing device ensures the mixing and prevents the chokingas a result of the production of by-products.

[0009] DE-AS 17 92 660 B2 relates to a process and a device for mixingand reacting an amine with phosgene resulting in an isocyanate. In thisprocess the amine and the phosgene are conducted coaxially and mixed,the two streams of amine and phosgene being ring-shaped or conical. Theyintersect at a sharp-cornered crossing and mixing location and they areaccellerated directly in front of, at, or behind the crossing locationfor entering into another reaction space in order to prevent a backflowof isocyanate into the stream of amine. This is made possible accordingto DE-AS 17 92 660 B2 by a device in which in the hollow part of theshaft of a T-shaped housing an inlet for the phosgene and in the hollowpart of the transverse bar a passage for the amine are provided. Acylindrical member is located in the passage for the amine, which closesone end of this passage and determines a reaction zone in the other endof this passage. The cylindrical member contains in its end, whichcloses the passage, an inlet for the amine in the housing. The housingcomprises a device for the adjustment of the velocity of flow of theamine. The end of this device, which points in the direction of areaction zone, comprises a section with a reducing profile, which has apredetermined distance from the end region, which points in thedirection of the reaction zone. This region directs the stream of amineentering into the cylindrical member in a certain angle away from theouter surface of the profile section and next to the reaction zone undera certain angle across the stream, which is directed to the passage. Thestream of amine which flows through the passage, which is narrowed bythe end region of the cylindrical member, meets the phosgene which flowsin from the reaction zone under a certain angle.

[0010] EP 0 830 894 A1 relates to a mixer-reactor and a process forcarrying out reactions, in particular the phosgenation of primaryamines. In this mixer-reactor, it is intended to prevent the blockage ofnozzles arranged rotationally symmetrically with respect to the mixingchamber by assigning a pin displaceable in the direction of the nozzleaxis. However, moving parts in phosgene-reacting reactors are potentialleakage points and should therefore be avoided as far as possible.

[0011] In view of the prior art solutions described, it is an object ofthe present invention to provide a process for the phosgenation ofamines which requires the use of less solvent and a smaller phosgeneexcess and in which less byproducts form.

[0012] We have found that this object is achieved, according to theinvention, if, in a process for mixing starting material streams in amixer for the phosgenation of amines, in which the reaction product isremoved in a closed loop and the starting material streams may containorganic solvents, main streams and/or part-streams of the startingmaterial come into contact with one another according to thecountercurrent principle.

[0013] In complete contrast to the opinion prevailing among thoseskilled in the art, the lowest degree of byproduct formation takes placewhen the starting material jets are fed directly toward one another. Bymeans of the chosen flow of the educts amine and phosgene, which areused in a liquid phase, a maximum mixing intensity can be achieved sincethe momentum of the two jets of the liquid phase of the educts cominginto contact with one

[0014] The invention is explained in more detail below with reference tothe drawing.

[0015]FIG. 1 shows the schematic configuration of a Y-mixer,

[0016]FIG. 2 shows the countercurrent mixing with axial discharge of theresulting reaction product,

[0017]FIG. 3 shows the countercurrent mixing with division of a startingmaterial stream into starting material main stream and starting materialpart-stream,

[0018]FIG. 4 shows the countercurrent mixing with an annular gapsurrounding a feed channel and having a shortened inner cylinder,

[0019]FIG. 5 shows the countercurrent mixing with radial discharge ofthe resulting reaction product,

[0020]FIG. 6 shows the surrounding of both starting material mainstreams with starting material part-streams of the respective othercomponent and

[0021]FIG. 7 shows the countercurrent mixing with generally inclineddischarge of the reaction product.

[0022] The diagram according to FIG. 1 shows the schematic arrangementfor a Y-mixer 13. A first starting material stream 1 and a secondstarting material stream 2 are fed in at a feed angle 4 which is greaterthan 90°. The resulting reaction product 3 is removed in a closed loopin a direction in space extending in the lower region of the mixerconfiguration 13 shown.

[0023]FIG. 2 shows the countercurrent mixing of two starting materialstreams with discharge of the reaction product essentially in the axialdirection.

[0024] In the embodiment shown in FIG. 2, a main stream 2.1 of thesecond starting material flows in inside a feed channel 6 while the mainstream 1.1 of the first starting material 1 flows in a feed channeldirectly countercurrently to the direction of flow of the main stream2.1. In the embodiment shown, both feed channels 6 and 7 are, forexample, symmetrical with respect to an axis 10 of symmetry. In themixing zone of the two main streams 2.1 and 1.1 of the startingmaterials 1 and 2, respectively, a discharge line 5 branches off, whichdischarge line is separated by a bounding wall 6.1 on the one hand andby a partition 7.1 to the feed channel 7 of the first starting materialmain stream 1.1. The discharge line 5 extends essentially in the axialdirection parallel to the shown feed channels 6 and 7 for the respectivestarting material main streams 1.1 and 2.1, respectively, and dischargesthe reaction product 3 formed from the mixing of the two startingmaterial main streams 1.1 and 2.1. In the embodiment shown in FIG. 2,the feed angle 4 of the two starting material main streams 2.1 and 1.1of the first and second starting materials, respectively, is about 180°,so that, as a result of the flow used here, the momentum of the twostreams when they come into contact with one another can be utilized forachieving a maximum mixing intensity and for producing maximum mixingenergy. On the partition 7.1 which separates the feed channel 7 of thefirst starting material main stream 1.1 from the discharge line 5 of thereaction product 3, the points critical for deposition of reactioncomponents are marked by the letters A and B.

[0025] The diagram according to FIG. 3 shows the countercurrent mixingwhen one of the starting material streams is divided into a main streamand a part-stream.

[0026] According to these embodiments of a mixing means for mixing twostarting material streams, the main stream 1.1 of the first startingmaterial flows into a feed channel 7, which however is separated fromthe discharge line 5, through which the reaction product 3 leaves themixing zone, not by a partition 7.1 according to FIG. 2 but by anannular gap 8. A part-stream 2.2 of the second starting material flowsthrough the annular gap 8 so that a phosgene, in the present case aphosgene excess, can be established in the region of the annular gaporifice 9 of the annular gap 8. By establishing the phosgene excess inthe region of the branch of the discharge line 5 from the feed channels6 and 7, it is possible to avoid a build up of deposits at that point ofthe annular gap A which is marked with A, i.e. a bounding wall of thedischarge line 5. It has been found that, by increasing the momentum ofthe main stream 1.1 of the first starting material stream in comparisonwith the momentum of the main stream 2.1 of the second starting materialstream, the build-up of deposits at point B, i.e. on the inside of thefeed channel 7, can be avoided. The avoidance of deposits at that pointof the feed channel 7 which is marked with B can be effected byincreasing the momentum of the main stream 1.1 of the first startingmaterial regardless of whether this component is present in excess of inless than the stoichiometric amount.

[0027] Furthermore, in the embodiment shown in FIG. 3, the feed channels6 and 7 for the main stream 1.1 of the first starting material and themain stream 2.1 of the second starting material are symmetrical withrespect to an axis 10 of symmetry. In addition to a rotationallysymmetrical design of the feed channels, it is of course also possibleto realize other cross-sections in them.

[0028]FIG. 4 shows the countercurrent mixing with an annular gapsurrounding a feed channel and having a shorter inner cylinder.

[0029] In the embodiment shown in FIG. 4, an annular gap 8 is present—ina manner roughly comparable with the embodiment according to FIG.3—between the discharge line 5 of the reaction product 3 and the feedchannel 7 for the main stream 1.1 of the first starting material. Apart-stream 2.2 of the second starting material is fed through theannular gap 8 to the mixing zone of the two main streams 1.1 and 2.1 ofthe starting materials 1 and 2, respectively, so that a phosgene, in thepresent case a phosgene excess, is present in the mixing zone. Byestablishing the phosgene excess by feeding a phosgene part-stream 2.2via annular gap 8, it is possible to avoid deposits, marked with A, inthe region of the discharge line 5, while deposits can be effectivelyprevented at the point marked with B, in the region of the annular gaporifice 9, by the shortened design of that surface of the feed channel 7which faces the axis 10 of symmetry. In the embodiment according to FIG.4, the wall 6.1 bounding the discharge line is shown only schematically.In order to optimize the flow conditions, the transition of the feedchannel 6 into the discharge line and the subsequent discharge line canbe designed with well rounded edges offering very little resistance toflow.

[0030] It should also be stated that elements generating angularmomentum can be installed in the feed channels 6 for the main stream 2.1of the second starting material and in the feed channel 7 for the mainstream 1.1 of the first starting material. During the mixing, the mixingenergy liberated in the mixing zone during reduction of the angularmomentum can be used for accelerating the mixing process. As an elementgenerating angular momentum, it would be possible, for example, tointroduce a twisted ribbon or spiral into the respective feed channels 6and 7 for the main streams 2.1 and 1.1 of the two starting materials.

[0031] The diagram according to FIG. 4 shows the countercurrent mixingof two starting material streams with radial discharge of the reactionproduct.

[0032] In the embodiment shown in FIG. 5, the feed channels 6 and 7 forthe main streams 2.1 and 1.1 of the two starting materials 1 and 2,respectively, are each rotationally symmetrical with respect to an axis10 of symmetry. In the configuration shown in FIG. 5, the feed channel 6for the main stream 2.1 of the second product has a greater diameter 16compared with the feed channel 7 for the main stream 1.1 of the firststarting material. The two feed channels 6 and 7 open into a radialdischarge line which is common to both and is arranged exactlyperpendicularly relative to the axis 10 of symmetry of the feed channels6 and 7 and permits perpendicular discharge of the reaction product.

[0033] The diagram according to FIG. 6 shows countercurrent mixing oftwo starting material streams where both starting material main streamsare surrounded by starting material part-streams of the respective othercomponent.

[0034] Analogously to the embodiments described in connection with FIG.3 and with FIG. 4, the discharge line 5 through which the reactionproduct 3 leaves the mixing zone is separated in the embodimentaccording to FIG. 6 by an annular gap 8. The annular gap 8 is bounded ineach case by an outer pipe 11 and an inner pipe 12 which, in theembodiment shown, open into the annular gap orifice, both outer pipe andinner pipe being of equal length and feeding a part-stream 2.2 of thesecond starting material to the mixing zone. In the embodiment accordingto FIG. 6, moreover, the outer wall of the feed channel 6 is moreover inthe form of a further annular gap 17. The annular gap consists of anouter pipe 18 and an inner pipe 19 and opens into the annular gaporifice 9 of the first annular gap 8 opposite, in the annular gaporifice 20, in the mixing zone of the two main streams 2.1 of the secondstarting material and 1.1 of the first starting material fed toward oneanother. The outer pipe 18 of the further annular gap 17 becomes thebounding wall 6.1 of the discharge line 5 for the reaction product 3, itbeing possible for the transitions to be designed with rounded edgesfavoring the flow conditions.

[0035] In the embodiment according to FIG. 7, countercurrent mixing witha discharge line, inclined at an angle, for the reaction product isshown.

[0036] In this embodiment according to FIG. 7, too, a main stream 2.1 ofa second starting material is fed through a feed channel 6 to a mixingzone to which a main stream 1.1 of the first starting material is fedlikewise through the feed channel 6, bounded by the outer wall 15. Inthe region of the mixing zone, a discharge line 5 for the reactionproduct 3 branches off and can be arranged at an angle α=30° withrespect to the axis 10 of symmetry. In addition to the inclination ofthe discharge line 5, shown in FIG. 7, further intermediate formsbetween an axial discharge of the reaction products 3 according to thediagram in FIGS. 2 and 3 and the radial discharge 14 according to FIG. 5are possible.

[0037] By means of the mixer configurations shown in the embodimentsdescribed above, mixing can be carried out with particularly effectiveutilization of the kinetic energy of the fluid streams. The method ofmixing results in particularly thorough contact between startingmaterials, since the energy inherent in the starting material jets canbe completely converted into mixing energy. The resulting high mixingintensities very substantially suppress byproduct formation and, bymeans of the novel process and the apparatus proposed according to theinvention for mixing two streams, permit the advantages of highoperational safety, avoidance of moving parts and achievement of highyields. Large phosgene excesses and high solvent contents in which thephosgene or the amines to be reacted have to be dissolved can beavoided, which is advantageous for subsequent working-up of the startingmaterials of the reaction product. As an example, it may be stated that420 kg/h of 2,4-toluenediamine (TDA) were premixed as a solution in 2450kg/h of o-dichlorobenzene (ODB) and sprayed together with 8100 kg/h of a65% strength phosgene solution into a T-mixer. The entrance diameters ofthe T-mixer were chosen so that a mean entry rate of the phosgene andamine solution jets of about 10 m/s resulted. After clear phosgenationand working up by distillation, a yield of 96.4% was obtained.

[0038] In the case of identical flow rates and entry velocities and theuse of a Y-mixer 13 with a feed angle 4 of about 120° between the twofeeds, a yield of 95.3% was obtained after clear phosgenation andworking up by distillation. Likewise in the case of identical flow ratesand entry velocities and the use of a countercurrent mixer with radialdischarge 14 of the reaction product 3, a yield of 97.4% was obtainedafter clear phosgenation and working up by distillation. List ofreference numerals 1 First starting material stream 1.1 Main stream 1.2Part-stream 2 Second starting material stream 2.1 Main stream 2.2Part-stream 3 Reaction product 4 Feed angle 5 Discharge line 6 Feedchannel for starting material 2 6.1 Bounding wall 7 Feed channel forstarting material 1 7.2 Partition 8 Annular gap 9 Annular gap orifice 10Axis of symmetry 11 Outer pipe 12 Inner pipe 12.1 Shortened inner pipe13 Y-mixer configuration 14 Radial discharge 15 Outer wall of startingmaterial channel 7 16 Outer wall of starting material channel 5 17Further annular gap 18 Outer pipe 19 Inner pipe 20 Annular gap orifice

We claim:
 1. A process for mixing starting material streams in a mixer(13) for the phosgenation of amines, in which the reaction product (3)is removed in a closed loop and the starting material streams (1, 2) maycontain organic solvents, wherein main streams (1.1, 2.1) and/orpart-streams (1.2, 2.2) of the starting materials (1, 2) come intocontact with one another according to the countercurrent principle undera feed angle (4) greater than 90° C. and wherein the momentum of onemain stream (1.1, 2.1) is higher than the momentum of the other mainstream (1. 1,
 2. 1).
 2. A process as claimed in claim 1, wherein thestarting material main streams (1.1, 2.1) are fed in surrounded bystarting material part-streams (1.2, 2.2).
 3. A process as claimed inclaim 2, wherein the main stream (1.1) of the first starting material issurrounded by a part-stream (2.2) of the second starting material.
 4. Aprocess as claimed in claim 2, wherein the main stream (2.1) of thesecond starting material is surrounded by a part-stream (1.2) of thefirst starting material.
 5. A process as claimed in claim 1, wherein theratio of the momentum of the main stream (1.1) of the first startingmaterial to that of the main stream (2.1) of the second startingmaterial is ≧1.
 6. A process as claimed in claim 1, wherein the reactionproduct (13) is discharged through an radial discharge line (14).
 7. Aprocess as claimed in claim 1, wherein the reaction product (13) isdischarged through an axial discharge line (5).
 8. A process as claimedin claim 1, wherein the discharge line (5) is oriented in an angularrange of from 0 to 90°, relative to an axis (10) of symmetry. 9.Apparatus for mixing two starting material streams (1, 2) which are fedto a mixing configuration (13), wherein main streams (2.1, 1.1) of thestarting materials (1, 2) are transported in feed channels (6, 7) whichare directed toward one another and from which a discharge line (5, 14)branches off.
 10. Apparatus for mixing as claimed in claim 9, wherein atleast one of the feed channels (6, 7) is surrounded by an annular gap(8, 17).
 11. Apparatus for mixing as claimed in claim 10, wherein theorifices (9, 20) of the annular gaps (8, 17) open in the region of thedischarge line (5).
 12. Apparatus for mixing as claimed in claim 9,wherein the annular gaps (8, 17) should have a shortened wall (12.1) inthe region of the mouth (9, 20).